As described in commonly assigned U.S. Pat. No. 5,320,643 to Roline et al., incorporated herein by reference, a cardiac pacemaker implantable pulse generator (IPG) is an electrical device used to supplant some or all of an abnormal heart's natural pacing function by delivering appropriately timed electrical stimulation signals designed to cause the myocardium of the heart to contract or "beat", i.e. to "capture" the heart. Stimulation pulses provided by implanted pacemakers usually have well-defined amplitude and pulse width characteristics which can be adjusted by remote programming and telemetry equipment to meet physiologic and device power conservation needs of the particular patient.
For state-of-the-art pacemakers, the rate at which stimulation signals are delivered may be variable, and such variation may occur automatically in response to detected changes in a patient's level of physical activity. Such rate-responsive or activity-responsive pacemakers depend on physiologically-based signals, such as signals from sensors which measuring naturally-occurring (intrinsic) cardiac electrical activity, or which measure the pressure inside the patient's ventricle. Such physiologically-based signals provide information regarding cardiac function and the need for pacemaker intervention, and thus are useful for determining a patient's metabolic demand for oxygenated blood.
One popular method for measuring a patient's demand for oxygenated blood is to monitor the patient's level of physical activity by means of a piezoelectric, microphone-like transducer mounted within and against the IPG can. A pacemaker which employs such a method is disclosed in U.S. Pat. No. 4,485,813 to Anderson et al.
In typical prior art rate-responsive pacemakers, the pacing rate is determined according to the output from an activity sensor. The pacing rate is variable between a predetermined maximum and minimum level, which may be selectable by a physician from among a plurality of programmable upper and lower rate limit settings. When the activity sensor output indicates that the patient's activity level has increased, the pacing rate is increased from the programmed lower rate by an incremental amount which is determined as a function of the output of the activity sensor. That is, the rate-responsive or "target" pacing rate in a rate-responsive pacemaker is determined as follows: EQU TargetRate=ProgrammedLowerRate+f(SensorOutput)
where f is typically a linear or monotonic function of the sensor output.
As long as patient activity continues to be indicated, the pacing rate is periodically increased by incremental amounts until the rate computed according to the above formula is reached (or until the programmed upper rate limit is reached, whichever is lower). In this way, an elevated pacing rate (i.e., one higher than the programmed lower rate limit) may be sustained during periods of patient activity. When patient activity ceases, the pacing rate is gradually reduced, until the programmed lower rate limit is reached.
For any of the known rate-responsive pacemakers, it is clearly desirable that the sensor output correlate to as high a degree as possible with the actual metabolic and physiologic needs of the patient, so that the resulting rate-responsive pacing rate may be adjusted to appropriate levels. A piezoelectric activity sensor can only be used to indirectly determine the metabolic need. The physical activity sensed by a piezoelectric transducer may in some cases be influenced by upper body motion. Therefore, an exercise that involves arm motion may provide signals that are inappropriately greater than the metabolic need. Conversely, exercises that stimulate the lower body only, such as bicycle riding, may provide a low indication of metabolic need while the actual requirement is higher.
To address these perceived disadvantages in the prior art, it has been proposed to utilize other physiologically-based parameters in assessment of a patient's metabolic demand. Respiratory minute ventilation (V.sub.E) has been demonstrated clinically to be a parameter that correlates directly to the actual metabolic and physiologic needs of the patient. Respiratory minute ventilation is defined by the equation: EQU V.sub.E =RR.times.TV
where RR=respiration rate in breaths per minute, and TV=tidal volume in liters. Clinically, the measurement of V.sub.E is performed by having the patient breathe directly into a device that measures the exchange of air and computing the total volume per minute. The direct measurement of V.sub.E is not practical with an implanted device. However, measurement of the impedance changes of the thoracic cavity can be implemented with an implanted pacemaker, and transthoracic cardiac impedance has been shown to correlate well with V.sub.E. A pacemaker that is provided with impedance measurement capabilities is disclosed in U.S. Pat. No. 4,702,253 to Nappholz et al. The magnitude of the change of the impedance signal corresponds to the tidal volume and the frequency of change corresponds to respiration rate. Thus, measurement of cardiac impedance can be used as one method for obtaining V.sub.E data.
In practice, cardiac impedance can be measured through assessment of the impedance present between two or more cardiac electrodes, such as the electrodes otherwise used for pacing and/or sensing in connection with a cardiac pacemaker. In particular, it has been shown that cardiac impedance can be measured by delivering constant-current excitation pulses between two "source" electrodes, such that the current is conducted through some region of cardiac tissue. The voltage differential between two "recording" electrodes can then be measured to ascertain the impedance as reflected by the voltage differential arising from the conduction of the excitation current pulses through the tissue.
In U.S. Pat. No. 4,721,110 to Lampadius, there is described a rheographic arrangement for a cardiac pacemaker in which the base pacing rate of the pacemaker is determined, in part, by a rheographically derived respiration rate signal. Correlation of breathing and intrathoracic pressure fluctuations with impedance of blood in the heart is also recognized in U.S. Pat. No. 4,884,576 to Alt, which describes the measurement of impedance between two electrodes. According to the '576 patent, low-pass filtering of the impedance signal yields a signal from which the patient's respiratory rate can be derived, while high-pass filtering of the same signal yields a signal from which the patient's cardiac function can be observed.
There are currently several commercially available, implantable, rate-responsive IPGs which employ rheographic techniques to adjust the pacing rate in response to metabolic needs. For example, the Biorate IPG manufactured by Biotec International, Bologna, Italy, uses a bipolar rheographic arrangement to monitor the patient's respiration rate. The Meta-MV IPG manufactured by Telectronics, Inc., Englewood, Colo., uses a tripolar rheographic arrangement to monitor the patient's metabolic demand for oxygenated blood. The Precept IPG manufactured by CPI, St. Paul, Minn., uses a tetrapolar rheographic configuration to monitor the patient's pre-ejection interval (PEI), stroke volume, and heart tissue contractility.
The Legend Plus.TM. IPG, manufactured by Medtronic, Inc., Minneapolis, Minn. and currently undergoing clinical trials in the United States is another example of an implantable pacemaker which employs rheography in support of its rate-response function. The Legend Plus.TM. IPG delivers a biphasic excitation signal between the pulse generator's canister (serving as an indifferent electrode) and a ring electrode of a transvenous pacing/sensing lead. Impedance sensing in the Legend Plus.TM. IPG is carried out between the lead's tip electrode and the pulse generator canister. The Legend Plus.TM. impedance measuring circuitry generates an impedance waveform in which both respiration and cardiac systole are reflected. This waveform is used by the pacemaker's circuitry to derive a minute ventilation value V.sub.E, as defined above. The Legend Plus.TM. IPG periodically assesses a patient's V.sub.E, and adjusts its base pacing rate up or down in accordance with the metabolic demand reflected in the V.sub.E value. Various aspects of the Legend Plus.TM. IPG are described in greater detail in commonly assigned U.S. Pat. No. 5,271,395 to Wahlstrand et al., incorporated by reference herein in its entirety.
Another disclosure which relates to the use of rheography in connection with an implanted device can be found in co-pending U.S. patent application Ser. No. 08/233,901 filed on Apr. 28, 1994, in the name of Wahlstrand et al. entitled METHOD AND APPARATUS FOR SENSING OF CARDIAC FUNCTION, which proposes a method and apparatus for obtaining an impedance waveform. The Wahlstrand et al. application, which relates to the use of a specialized lead for improving the quality of an impedance waveform like that utilized in the aforementioned Legend Plus.TM. IPG, is hereby incorporated by reference herein in its entirety.
Yet another disclosure relating to the use of rheography in connection with implantable devices can be found in co-pending U.S. patent application Ser. No. 08/277,051 filed on Jul. 19, 1994, in the name of Gianni Plicchi et al., entitled TIME-SHARING MULTI-POLAR RHEOGRAPHY.
As noted above, the utilization of a piezoelectric transducer in a cardiac pacemaker provides a useful but only an indirect indication of a patient's actual level of physical activity, and thus allows for the possibility of false positive or false negative indications of elevated levels of a patient's metabolic demand. The above-noted problem associated with upper body movement is one example of this.
Similarly, the measurement of intracardiac impedance using rheographic techniques provides a useful but somewhat indirect indication of a patient's respiration and cardiac rates, and therefore also allows for the possibility of error in determining a patient's metabolic need. It has been shown that the use of transthoracic impedance to indicate minute ventilation levels has the potential for false positive indications of elevated metabolic demand levels, due to upper body myopotential interference and postural changes. Furthermore, slow-acting physiologic parameters such as transitory blood chemistry changes can also impact impedance measurement.
In addition, basing pacing rate solely on respiratory minute ventilation measurements does not always provide an optimum pacing rate increase at the onset of exercise. Tidal volume (TV) and respiration rate (RR) levels have an inherent physiological time delay due to the response of the CO.sub.2 receptors and the autonomic nervous system. An increase in V.sub.E can lag behind the need for increased cardiac output.
On the other hand, activity signals derived from a piezoelectric transducer do not typically exhibit this same time delay phenomenon at the onset of exercise. Moreover, minute ventilation signals derived from transthoracic impedance measurements tend to be more appropriately proportional to a wider variety of types of exercise (e.g., bicycling, walking, running, etc . . . ) than piezoelectric sensor signals tend to be. In this regard, piezoelectric activity signals and transthoracic impedance measurements are mutually complementary in their efficacy in establishing a patient's level of metabolic demand. That is, the potential limitations of each type of sensing are different. This suggests that a combination of activity sensing using a piezoelectric transducer and minute ventilation sensing using rheographic techniques would provide an improved method of accurately tracking a patient's level metabolic demand. Such an approach is set forth in the above-referenced '813 application and in commonly assigned U.S. Pat. No. 5,441,524 to Rueter et al., incorporated by reference herein.
Similarly, the combination of two or more rate control parameters (RCPs), e.g. piezoelectric activity sensors and blood pressure sensors, has also been proposed in commonly assigned U.S. Pat. No. 5,154,170 to Bennett et al., incorporated by reference herein. The '170 patent sets forth an optimization routine for assigning weighting values to the enabled sensor outputs for deriving the appropriate pacing rate in a variety of circumstances.
In virtually all of the approaches, it is necessary to rely on additional components and circuitry, e.g. additional subcutaneous leads or electrodes and/or a current signal generator for making the impedance change measurements which consumes more energy.
In U.S. Pat. No. 4,763,646 to Lekholm, a heart sound detector is also proposed to be mounted in one or more pacing leads arranged in or about the heart or to be mounted in the IPG case for acoustically sensing heart sounds transmitted through a fluid filled lumen. The use of a pressure sensor, microphone or accelerometer is proposed for the heart sound detector.
In one further approach set forth in U.S. Pat. No. 5,063,927 to Webb, the output signal of a piezoelectric activity sensor mounted in the IPG can is filtered to derive an activity signal and a respiration rate signal in lieu of using the rheography technique described above. Respiratory minute ventilation is not described and may be difficult to distinguish from other sources of in-band false signals detected by a piezoelectric activity sensor mounted to the IPG can.
A need exists therefore for a body implantable, durable, long lived, simple and low power sensor for accurately detecting both the respiration rate and tidal volume of the patient for use in determining the physiologic need for cardiac output and automatically adjusting the pacing rate.